The present invention relates to a sheet stock feed line in which ferrous or nonferrous sheet stock is fed by a feeder to a tool for cutting, press, or similar machining operation.
FIG. 1 shows a widely used sheet stock feed line in which sheet stock is fed a predetermined length by a feeder for cutting by a shear. In this prior art example, ferrous or nonferrous sheet stock 12 rolled on an uncoiler 11 is unrolled and straightened or flattened by a leveler 13 which serves also as a feeder, thereafter being fed to a shear 14. The leveler 13 is driven by a motor 16 through a reducer 15. Pulses corresponding to the revolution of the motor 16 are generated by a rotary encoder 17 and are supplied to a numerical controller 18, which responds to the pulses to effect numerical control of the motor 16 to feed the sheet stock 12 a predetermined length (i.e. a length into which the sheet stock 12 is desired to be cut). After the feed of the sheet stock 12 by this numerical control, a cut command is issued to the shear 14, causing it to cut the sheet stock 12 while the latter is stopped. Upon completion of cutting, the next feed command is provided to the numerical controller 18. By repeating this procedure, the sheet stock 12 is successively cut into preset lengths.
Recently a surface treated sheet, typically, a colored steel sheet, has come into common use. The surface treated sheet is marred easily when the leveler 13 is stopped and restarted. This is called a roll mark of the leveler, which is said to be caused by the slipping between rolls of the leveler and the sheet stock when the leveler is started after being stopped. By firmly screwing down the leveler to bring its rolls into tight contact with the sheet stock, the frictional force developed therebetween will prevent the slipping to some extent. For most of recent surface treated sheets, however, the force for screwing down the leveler must be reduced because such sheets are easily injured and marred with a roll mark.
FIG. 2 shows a conventional sheet stock feed line designed to prevent sheet stock from being marred with a roll mark. In this feed line, the sheet stock 12 is fed continuously, without stopping the leveler 13, at such a constant rate that a loop (i.e. the slack or sag in the sheet stock) 25 described later stably remains at substantially the same position. The sheet stock 12 fed by the leveler 13 is fed by a second feeder 19 to the shear or press 14. The feeder 19 is driven by a motor 22 through a reducer 21. A rotary encoder 23 coupled with the motor 22 generates pulses corresponding to its revolution, and the pulses are applied to a numerical controller 24. The numerical controller 24 controls the motor 22 to feed the sheet stock 12 by a preset feed length. When the sheet stock 12 is fed the preset length by this numerical control, the shear or press 14 is actuated to cut or press the sheet stock 12 in the standstill period of the sheet stock 12. Upon completion of the working, the next feed command is issued to the numerical controller 24, thereafter repeating the same operation.
The feeder 19 repeatedly suspends the feed of the sheet stock 12, but since the leveler 13 always feeds the sheet stock 12, the loop 25 of the sheet stock 12 between the leveler 13 and the feeder 19 goes down while the feed of the sheet stock 12 by the feeder 19 is suspended. The loop 25 goes up by the acceleration of the feeder 19. A pit 26 is provided for receiving the loop 25.
The feed line shown in FIG. 2 calls for the provision of the pit 26 of a size corresponding to a maximum feed length of the sheet stock 12, resulting in increased line length. In order that the flatness of the sheet stock 12, once leveled by the leveler 13, may be prevented from degradation by the formation of the loop 25, it is necessary that the radius of curvature of the loop 25 be large. Hence, the line length inevitably increases.
FIG. 3 shows another conventional sheet stock feed line which is relatively short and does not mar sheet stock. The leveler 13 feeds the sheet stock 12 to a rotary shear 31 at a feed rate dependent on the length into which the sheet stock 12 is to be cut. The rotary shear 31 is driven by a motor 33 through a reducer 32. Pulses corresponding to the revolution of the motor 33 are generated by a rotary encoder 34 coupled therewith and are applied to a numerical controller 35. The numerical controller 35 is also supplied with pulses from a rotary encoder 28 coupled with length measuring rolls 27. The numerical controller 35 controls the motor 33 to drive the rotary shear 31 to make one turn for each preset feed of the sheet stock 12 so that upper and lower cutting edges of the rotary shear 31 mesh with each other to cut the sheet stock 12 at the end of the preset feed thereof. In this instance, the rotary shear 31 is driven at about the same speed as the feed rate of the sheet stock 12 during the shearing work.
In this prior art example, the rotary shear 31 must be sufficiently large and heavy for the shearing work, and consequently, the motor 33 for driving such a shear also needs to be large in power. Accordingly, this feed line inevitably becomes expensive as a whole.
Furthermore, this conventional feed line is not suitable for cutting sheet stock into lengths shorter than the circumference of the addendum circle of the rotary shear (700 mm or so in many cases, represented by .pi.D, where D is the diameter of the addendum circle), because the feed of sheet stock must be rapidly accelerated and then quickly decelerated for each cutting operation. It is therefore almost impossible to cut sheet stock into a length of, for example, 300 mm. Additionally, the feed rate must be decreased markedly, but at too low feed rates the flywheel effect of the rotary shear is so small that sheet stock cannot be cut only with the force of the motor driving the rotary shear. The feed line shown in FIG. 3 can be used for cutting sheet stock into medium and long sizes, but plants which have direct dealing with end users are required to fill their demands for small numbers of sheets of a large variety of sizes.